In recent years, a solar cell directly converting sunlight energy to electrical energy has particularly received attention as a next-generation energy source to replace fossil fuels because the solar cell puts few burden on the global environment. Solar cells in practical use at present mainly include a solar cell using a crystalline silicon substrate and a thin-film silicon solar cell are. In both of these solar cells, efforts are being made to reduce cost per power output by increasing efficiency in photoelectric conversion.
It is difficult, however, to popularize the solar cell using a crystalline silicon substrate because fabrication of the crystalline silicon substrate is expensive. On the other hand, the thin-film silicon solar cell has a problem in that the manufacturing cost is high because many types of gasses for manufacturing the semiconductor and complicated devices must be used in the manufacturing process. Therefore, both of the solar cells have not solved the problem of high manufacturing cost so far.
Therefore, as a different type of solar cell from the aforementioned two types of solar cells, Japanese Patent Laying-Open No. 01-220380 (hereinafter also referred to as “Patent Literature 1”) proposes a dye-sensitized solar cell in which photoinduced electron transfer of a metal complex is applied. In this dry-sensitized solar cell, a glass substrate having a porous electrode on a surface thereof and a counter electrode are prepared, and the porous electrode has a light sensitizing dye adsorbed thereon to provide an absorption spectrum in a visible light region.
When this dye-sensitized solar cell is irradiated with light from the transparent electrode side, the light sensitizing dye contained in a photoelectric conversion layer absorbs the light to generate electrons. Then, the generated electrons transfer from one electrode through an external electrical circuit to the opposite electrode. The transferred electrons are carried by ions in an electrolyte to return to the photoelectric conversion layer. By such a series of electron transfer, electrical energy can be continuously extracted from the dye-sensitized solar cell.
The dye-sensitized solar cell disclosed in Patent Literature 1, however, has a structure with the electrolyte injected between the electrodes on the two glass substrates. Therefore, although a prototype of a small-area dye-sensitized solar cell can be made, it is difficult to fabricate a large-area dye-sensitized solar cell such as a dye-sensitized solar cell measuring 1 m per side.
In addition, by increasing the area of one dye-sensitized solar cell, a current obtained from the dye-sensitized solar cell increases in proportion to the area. However, the resistance in the in-plane direction of the transparent electrode also increases, which causes an increase in the internal serial electrical resistance. As a result, a fill factor (FF) in the current-voltage characteristics at the time of photoelectric conversion as well as a short-circuit current decrease, and the photoelectric conversion efficiency deteriorates.
Therefore, as an attempt to solve the problems as described above, a dye-sensitized solar cell has also been proposed in which a plurality of dye-sensitized solar cells are connected in series. In this dye-sensitized solar cell, an electrode (conductive layer) of the dye-sensitized solar cell and an electrode (counter electrode) of an adjacent dye-sensitized solar cell are electrically connected, thereby suppressing an increase in internal serial electrical resistance (refer to, e.g., Japanese Patent National Publication No. 11-514787 (International Publication No. WO97/16838 (hereinafter also referred to as “Patent Literature 2”)), Japanese Patent Laying-Open No. 2001-357897 (hereinafter also referred to as “Patent Literature 3”), and Japanese Patent Laying-Open No. 2002-367686 (hereinafter also referred to as “Patent Literature 4”)).